1. Field of the Invention
The present invention relates to a charge pump, a phase/frequency detector and related methods.
2. Description of the Prior Art
A charge pump (CP) is an essential component in a phase locking system for performing phase/frequency synchronization. A conventional CP usually suffers from nonlinearity due to a dead zone or undesired noise from a reference spur. In order to alleviate the dead zone issue, an offset current is added to provide a constant current to avoid the dead zone, making the performance of the CP more linear when it is phase-locked. Please refer to FIG. 1, which is an exemplary diagram of a conventional CP 1000 coupling to a voltage controlled node VCTRL via a loop filter 2000. The conventional CP 1000 is for conducting a current ICP via an output terminal N, and includes a sourcing element 1100, a draining element 1200 and an offset element 1300. The sourcing element 1100 and the draining element 1200 are dominated by control signals UP, UPB and DN, DNB, respectively, wherein the control signals UPB, DNB are inverses of the control signals UP, DN, respectively. When the phase locking system containing the conventional CP 1000 is in a phase-locked state, the sourcing element 1100 will provide an additional current to cancel out the constant current injected by the offset element 1300. In this way, the nonlinearity caused by the dead zone issue can be alleviated.
Although the addition of the offset current can successfully overcome the dead zone problem, the reference spur issue will still remain. Please refer to FIG. 2, which is a timing diagram of the conventional CP 1000 when phase-locked. When the conventional CP 1000 is in the phase-locked state, a current IUP generated by the sourcing element 1100 is partially cancelled out by a current IDN drained from the draining element 1200, and the other current IUP is cancelled by a current Ioffset drained from the offset element 1300. As a result, a total current ICP from the conventional CP 1000 appears to source an impulse current into a following circuit and then keeps draining a constant current from the following circuit until a next reference period. Therefore, the following voltage controlled node VCTRL, e.g., a control node of a voltage controlled oscillator following the conventional CP 1000 via a loop filter, will experience a periodical modulation resulting in a reference spur.
In addition, please note that each electrical charge injected into or from the voltage controlled node VCTRL may produce undesired noise. Ideally, each current from the sourcing element 1100, the draining element 1200 and the offset element 1300 will cancel out the other currents when the phase-locked state is reached and therefore have no influence over the voltage controlled node VCTRL; nevertheless, in practical implementation, those existing electrical charges from all current sources will still impact the overall performance of the phase locking system. It is therefore an important issue to minimize the amount of current flow involved in the phase-locked state.